1. Field of the Invention
The present invention relates to a technique of detecting deterioration of a catalyst, disposed in a path of exhaust gas emitted from an engine used in a vehicle or the like, which catalyst particularly includes ceria serving as an oxygen storage agent and has NOx conversion capacity.
2. Description of the Related Art
In today's vehicle, a catalyst is disposed in a path of exhaust gas emitted from an engine so that exhaust gas is converted into a harmless form. In accordance with present progress in study on catalyst, various catalysts with a property of higher conversion capacity have been developed. However, conversion capacity of a catalyst is not permanently maintained, but is gradually reduced according to deterioration of the catalyst caused by usage of the catalyst. For this reason, considering from a viewpoint on prevention of environmental pollution, it is important to develop a technique for accuracy detection of a degree of catalyst deterioration as well as study on catalysts having high conversion capacity.
A Japanese patent Laid-open publication HEI 5-248227 (especially in FIG. 10), which is hereinafter called a reference 1, discloses one of conventional methods for deterioration detection of a catalyst. The technique of the reference 1 focuses on the fact that oxygen storage capacity of the catalyst. The oxygen capacity is associated with a degree of catalyst deterioration varies depending on the catalyst temperature. Further, the reference 1 focuses on the point that “new”, “not-new but good” and “deteriorated” catalysts possess respective different properties of variation in oxygen storage capacity especially in a temperature range of 300-550° C. Considering the above points, the reference 1 proposes that a value used for determination of catalyst deterioration is not fixed but is flexibly set in accordance with catalyst temperature.
However, the temperature range of 300-550° C., on which the reference 1 focuses, is lower than a temperature range at which a catalyst is adequately activated. In other words, the catalyst at the above temperature range is still partially activated and has hitherto low conversion efficiency.
In the meanwhile, the recent technology can heat a catalyst at an early stage, especially an adjacent catalyst disposed adjacent to an engine, to an activation temperature at which the catalyst is activated. Additionally, such an adjacent catalyst can continuously maintain high temperature because of closeness to the engine.
A catalyst is therefore in the above low temperature range in an extremely short period under a normal running state. Practically, determination of deterioration should be performed in a high temperature range in which the catalyst is adequately activated.
As shown in FIG. 10 of the reference 1, there is a small difference in oxygen storage capacities of “new” and “not-new but good” catalysts at a temperature of 550° C. or higher. Such a small difference makes the method in the reference 1 to accurately detect a degree of catalyst deterioration.
The above conventional detection method cannot be applied to a strict anti-deterioration standard which does not permit even a minor deterioration is made in order to realize high capability of exhaust-gas conversion.